1. Field of the Invention
The present invention relates to a method of and apparatus for generating a conversion curve employable for converting a reproduction characteristic of an image and more particularly to a technique suitable for automatic generation of the conversion curve in a process scanner and the like. The present invention may be applied to a method of systematically correcting color fogging caused only in a shadow part of a color original in a color image processor such as a color process scanner.
2. Description of the Related Art
In a color process scanner and the like, an image of a color original is color-separated and thereafter each color component is converted into a halftone dot signal to record a reproduced image. When color fogging is present in the original, color separation conditions are corrected in the judgement of an operator so that gray balance is ensured in a recorded image.
In a first conventional method for correcting the color separation conditions, different highlight and shadow density values are set for respective color components in a process for converting a density signal for each color component obtained by reading the original to fit a density range of a color separation device, that is, a normalization conversion process.
In a second conventional method, highlight and shadow density values are set in common to each color component, and halftone dot percentage for each color block is corrected in the stage where the halftone dot signal for each color block: yellow (Y), magenta (M), cyan (C) and black (K) is determined.
There are generally two problems in such conventional methods.
One problem is that a skilled operator is required for proper correction, because the color separation conditions are corrected fundamentally based on a manual operation controlled by an operator. This type of problem occurs not only in the correction of the color separation conditions but also in the normalization conversion in a monochrome image.
The other problem is one inherent in obtaining the aforesaid gray balance. In the two conventional methods, improvement is recognized in the case where a color fogging is present in low and middle density regions due to a deviation in the light-emission characteristic of a light source in photographing a subject for producing the original and in the case where the original is discolored as a whole.
It is however difficult to carry out appropriate correction through the conventional methods in the case where a color fogging caused only in a shadow part results from a difference between respective maximum values of color components in expressible density on an original film. Since uniform correction is carried out over a whole range of density in the conventional methods, the correction for removing the color fogging in the shadow part influences the low and middle density regions, and as a result the gray balance in the low and middle density regions is lost.
UCA/UCR (Under Color Add/Removal) function in the color separation device should be used to remove the color fogging only on the high density side while preventing such states. By using this function, only the halftone dot percentage of gray component having high density is adjustable for each color printer in relation to the gray component.
However, when the UCA/UCR function is used, the lower limit of the density range to be corrected must be set manually for each color printer. The decision of a correction amount is largely dependent on the operator's sense because the correction amount is also based on the darkest part in the original. That is, the work of removing the color fogging by means of the UCA/UCR function is not objective and systematic but temporary. Thus the correction amount of the color fogging by means of the UCA/UCR function cannot be controlled systematically and quantitatively, which causes a hindrance to automatic determination of color separation conditions.
In addition, since a UCA/UCR circuit is provided in the rear stage of a black-printer signal generation circuit in the color separation device, each circuit in the preceding stage of the UCA/UCR circuit must process a signal before gray balance correction, and establishment of the processing characteristic thereof is complicated.
Furthermore, a technique which handles together both gray balance correction in the case where the entire original is discolored and color fogging correction only in the shadow part has not been developed, and these corrections must be conducted separately in the conventional techniques.